It'sa type of finite differencing stencil. You should be able to learn about it in any finite difference textbook.

The idea is that finite difference stencils originate from Taylor series. By expanding a function, 'f' say, we can develop finite difference approximations to derivatives.

Upstream differencing means that the finite difference operators are calculated by using points that are up-stream.

An example is the first derivative of f with respect to x (where dx is the grid spacing). The derivative at the node 'i' can be written f_x(central)=(1/2)*(f(i+1)-f(i-1))/dx f_x(upstream)=(f(i+1)-f(i))/dx

Other operators are available; the order of the operator comes from the truncation error in the Taylor series.

QUICK is one type of higher-order upwind, and refers to the reconstruction used to generate face values of the field variables (although it can also be used on the flux variables directly). Such reconstructions are typically used in conjunction with Riemann type solvers, although you can also find higher order upwind reconstruction used in incompressible solvers. One can also formally develop upwind stencils following Bren's outline. Try googling for higher-order upwind CFD.